Method for forming heavy nitrogen-doped ultra thin oxynitride gate dielectrics

ABSTRACT

A method for forming an ultra thin gate dielectric for an integrated circuit device is disclosed. In an exemplary embodiment of the invention, the method includes forming an initial nitride layer upon a substrate by rapidly heating the substrate in the presence of an ammonia (NH 3 ) gas, and then re-oxidizing the initial nitride layer by rapidly heating the initial nitride layer in the presence of a nitric oxide (NO) gas, thereby forming an oxynitride layer. The oxynitride layer has a nitrogen concentration therein of at about 1.0×10 15  atoms/cm 2  to about 6.0×10 15  atoms/cm 2 , and has a thickness which may be controlled within a sub 10 Å range.

BACKGROUND

The present invention relates generally to semiconductor processing and,more particularly, to improved techniques for fabricating gatedielectrics.

As integrated circuits have become smaller and more densely packed, sohave the dielectric layers of devices such as field effect transistorsand capacitors. With the arrival of ULSI (Ultra Large Scale Integratedcircuit) technology and gate dielectrics of less than 15 angstroms (Å)in thickness, the use of silicon dioxide (SiO₂) as a traditional gatedielectric material becomes problematic.

In larger devices (e.g., where the gate oxide thickness is 40 Å ormore), leakage currents from a polysilicon gate electrode, through thegate oxide and into the device channel, are only on the order of about1×10⁻¹² A/cm². However, as the thickness of an SiO₂ gate dielectric isdecreased below 20 Å, the leakage currents approach values of about 1A/cm². This magnitude of leakage current, caused by direct tunneling ofelectrons from the polysilicon gate electrode through the gate oxide,results in prohibitive power consumption of the transistor(s) in theoff-state, as well as device reliability concerns over an extendedperiod of time.

Another problem with ultrathin SiO₂ gate dielectrics relates to thedoping of the polysilicon gate electrodes with a dopant material, suchas boron. Such doping is typically used to combat channel depletioneffects which cause voltage threshold (V_(t)) shifts and higherthreshold voltages. With an ultrathin SiO₂ gate dielectric, however, theboron dopant atoms can easily penetrate the SiO₂ layer and thereby causelarge V_(t) shifts and reliability problems themselves.

Accordingly, the nitrogen doping of gate dielectrics has become apreferred technique of semiconductor chip manufacturers. For gatedielectrics having a thickness range of about 15 Å to 20 Å, siliconoxynitride (SiO_(x)N_(y)) layers have replaced SiO₂ layers as the choiceof gate dielectric material. The beneficial effects of nitrogenincorporation into the dielectric are generally dependent upon theconcentration of the doping and the distribution of the doping profilerelative to both the Si/SiO₂ interface and the polysilicon gate/SiO₂interface. If properly carried out, the nitrogen doping reduces leakagecurrent and boron penetration, while minimizing or negating the impacton V_(t) and channel electron mobility.

Present nitridation techniques, however, do have certain drawbacksassociated therewith. For example, a rapid thermal annealing process(such as in the presence of N₂O or NO gas) by itself may not result in asufficiently high nitrogen content so as to faciliate the desiredreduction in leakage current. In the case of a plasma process, such asremote plasma nitridation (RPN), the possibility exists that the ionizedplasma species will cause damage to active devices formed on thesemiconductor wafer.

BRIEF SUMMARY

The above discussed and other drawbacks and deficiencies of the priorart are overcome or alleviated by a method for forming an ultra thingate dielectric for an integrated circuit device. In an exemplaryembodiment of the invention, the method includes forming an initialnitride layer upon a substrate and then re-oxidizing the initial nitridelayer, thereby forming an oxynitride layer. The oxynitride layer has anitrogen concentration therein of at least 1.0×10¹⁵ atoms/cm² and has athickness which may be controlled within a sub 10 Å range.

In a preferred embodiment, forming the initial nitride layer includesrapidly heating the substrate in the presence of an ammonia (NH₃) gas attemperature of about 650° C. to about 1000° C., and at a pressure ofabout 1 Torr to about 760 Torr. Re-oxidizing the initial nitride layerincludes rapidly heating the initial nitride layer in the presence of anitric oxide (NO) gas at temperature of about 650° C. to about 1000° C.,and at a pressure of about 1 Torr to about 760 Torr. The oxynitridelayer preferably has a nitrogen atom concentration of about 1.0×10¹⁵atoms/cm² to about 6.0×10¹⁵ atoms/cm².

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the exemplary drawings wherein like elements are numberedalike in the several Figures:

FIGS. 1(a)-(b) are schematic diagrams which illustrate the steps of aknown process of forming a nitrided gate dielectric material;

FIGS. 2(a)-(b) are schematic diagrams which illustrate the steps ofanother known process of forming a nitrided gate dielectric material;

FIG. 3 is a process flow diagram which illustrates a method for forminga gate dielectric for an integrated circuit device, in accordance withan embodiment of the invention;

FIGS. 4(a)-(c) are schematic diagrams which illustrate the steps shownin FIG. 3;

FIG. 5 is a graph which illustrates the resulting thicknesses of nitridelayers formed by the RTNH₃ process;

FIG. 6 is a graph which illustrates the initial nitrogen concentrationintroduced into the silicon nitride layer following the RTNH₃ process;and

FIG. 7 is a graph which illustrates the resulting oxynitride filmthickness and nitrogen concentration therein after re-oxidation of theinitial silicon nitride layer.

DETAILED DESCRIPTION

Referring initially to FIGS. 1(a) and 1(b), a known process of nitridinga gate oxide layer is illustrated. A silicon substrate 10 (e.g., eithera bare silicon wafer or a silicon wafer having a thin SiO₂ layerthereupon) is subjected to a thermal anneal in the presence of anitrogen containing gas, such as nitric oxide (NO). Alternatively, thegas may also be nitrous oxide (N₂O). Thereby, a layer of oxynitride(SiO_(x)N_(y)) film 12 is formed for use as a dielectric material, asshown in FIG. 1(b). One problem with layer 12, however, is therelatively low concentration of nitrogen atoms diffused therein. A lowernitrogen content dielectric is not as effective in reducing leakagecurrent from the gate conductor. In addition, an RTN₂O process by itselfdoes not easily lend itself to producing reliable gate dielectricthicknesses below 10 Å.

In FIGS. 2(a) and (b), there is shown another existing process offorming a nitrided gate dielectric. The silicon substrate 10 has aninitial oxide layer (SiO₂) 14 formed thereupon. The wafer is thensubjected to a plasma process, such as a remote plasma nitridation(RPN). As a result of the RPN process, the oxide layer 14 is convertedto a silicon oxynitride layer 12 having the general chemical compositionSiO_(x)N_(y), as shown in FIG. 2(b). While an RPN process generallyprovides an increased nitrogen concentration in the oxynitridedielectric layer, the formation of a consistent, ultra-thin layer (5-10Å) is still problematic. Moreover, the ionized plasma species generatedduring the process can cause damage to active devices formed on thesemiconductor wafer, as stated earlier.

Therefore, in accordance with an embodiment of the invention, apreferred process flow for a method 100 of forming a gate dielectric foran integrated circuit device is shown in FIGS. 3 and 4(a)-(c). In step102, a silicon substrate 200 is prepared with clean silicon surface forformation of the dielectric thereupon. The silicon surface is preferablyhydrogen-terminated and initially cleaned of any oxide materialthereupon. This may entail a preliminary cleaning step, such as by anozone plasma clean and a dilute hydroflouric acid (DHF) etch. In step104, an ultra-thin initial silicon nitride layer 202 is formed by rapidthermal processing in NH₃, thereby introducing a desired concentrationof nitrogen atoms into the substrate 200 surface. In step 106, thesilicon nitride layer 202 is re-oxidized by rapid thermal processing innitric oxide (NO). While NO is a preferred gas for the thermalre-oxidation process, O₂ may also be used. An oxynitride dielectriclayer 204 is thereby formed which has a desired nitrogen contentthickness. Depending on the specific time, temperature and processconditions of the above steps, the oxynitride layer 204 may be scaleddown below 10 Å in thickness.

More specifically, the initial silicon nitride layer (having the generalchemical composition Si₃N₄) is formed on the silicon surface by heatingin the presence of an ammonia (NH₃) gas at a temperature range of about650° C. to about 1000° C. at a pressure range of about 1-760 Torr. Forexample, an initial rapid thermal (RTNH₃) process at a temperature of900° C. and a pressure of 550 Torr for a duration of about 15 seconds isfound to grow a silicon nitride layer of about 8 Å. In a preferredembodiment, the RTNH₃ process is carried out in a single wafer, rapidthermal process chamber, especially when forming ultra thin films (<20Å). However, the semiconductor wafer may also be annealed in a batchprocessing tool such as a furnace.

FIG. 5 is a graph which illustrates the resulting thicknesses of nitridelayers formed by the RTNH₃ process at varying temperatures and times, ata pressure of 550 Torr. As can be seen, the nitride thickness ranges areabout 5.5 Å at 700° C. to about 8.2 Å at 900° C. for a 15 secondduration, and about 6.5 Å at 700° C. to about 12 Å at 900° C. for a 60second duration. FIG. 6 illustrates the initial nitrogen concentrationintroduced into the silicon nitride layer following the RTNH₃ process.As suggested by the graph, the concentration of nitrogen incorporationmay be tuned from about 1×10¹⁶ N atoms/cm² to about 6×10¹⁶ N atoms/cm²,depending on the temperature, time and pressure conditions.

Next, the initial silicon nitride layer is subjected to a re-oxidationprocess by rapid thermal processing in nitric oxide (NO) gas to producean oxynitride dielectric layer. This RTNO process may be carried out ata temperature range of about 650° C. to about 1000° C. at a pressurerange of about 1-760 Torr. In the above example, the RTNO processimplemented at 950° C., at a pressure of 740 Torr, for a duration ofabout 30 seconds resulted in an oxynitride layer of about 14.5 Å. Thehydrogen incorporated into the silicon nitride layer during the ammonianitridation is reduced by the oxygen present in the RTNO step anddiffuses out at the high oxidation temperature.

Referring now to FIG. 7, there is shown a graph which illustrates theresulting oxynitride film thickness and nitrogen concentration thereinafter the re-oxidation of the initial silicon nitride layer. Forexample, the darkened square on the left portion of the graph representsthe film thickness of the initial silicon nitride layer formed by RTNH₃at 800° C. for 15 seconds. The initial thickness, about 6.5 Å, isincreased to about 12.5 Å after RTNO re-oxidation (as indicated by thehollow square on the left portion of the graph). Moreover, the RTNOprocess does not substantially reduce the nitrogen concentration in theresulting oxynitride layer, as also shown in FIG. 5. Again, the initialsilicon nitride layer formed by RTNH₃ at 800° C. for 15 seconds had anitrogen concentration of about 3.04×10¹⁵ atoms/cm², which was onlyreduced to about 2.99×10¹⁵ atoms/cm² after re-oxidation.

For longer process times (e.g., 45 seconds), as seen on the rightportion of the graph in FIG. 5, the nitrogen concentration decrease issomewhat greater after re-oxidation, but not significantly so. Forexample, an initial silicon nitride layer formed by RTNH₃ at 900° C. hada thickness of about 11.5 Å and a nitrogen concentration of about5.54×10¹⁵ atoms/cm² before re-oxidation (as indicated by the darkeneddiamond). Afterward, the resulting oxynitride layer had a thickness ofabout 14.5 Å and a nitrogen concentration of about 5.01×10¹⁵ atoms/cm².

As a result of the above described processes, a reliable, ultra-thingate dielectric with good film uniformity may be achieved. Furthermore,the process may be carried out in-situ, without plasma damage and/ormetallic contamination. Other improved device characteristics are alsorealized. In a CMOS device employing both p-type and n-type transistors,the change in threshold voltage (as compared to a conventional 13 Å RTNOprocess only) is about 80 mV lower for an NFET device and about 200 mVhigher for a PFET device. In addition, as compared to RTNO, the NFETdevice shows about a 0.7 A gate tunneling reduction with the RTNH₃+RTNOprocess.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

What is claimed is:
 1. A method for forming a gate dielectric for anintegrated circuit device, the method comprising: forming an initialnitride layer upon a substrate by rapidly heating said substrate in thepresence of an ammonia (NH₃) gas; and re-oxidizing said initial nitridelayer, thereby forming an oxynitride layer; wherein said oxynitridelayer has a nitrogen concentration therein of at least 1.0×10¹⁵atoms/cm², and a thickness of about 5 Å to about 20 Å.
 2. The method ofclaim 1, wherein: said forming an initial nitride layer is carried outat temperature of about 650° C. to about 1000° C., and at a pressure ofabout 1 Torr to about 760 Torr.
 3. The method of claim 1, wherein saidre-oxidizing said initial nitride layer further comprises: rapidlyheating said initial nitride layer in the presence of a nitric oxide(NO) gas.
 4. The method of claim 3, wherein: said re-oxidizing saidinitial nitride layer is carried out at temperature of about 650° C. toabout 1000° C., and at a pressure of about 1 Torr to about 760 Torr. 5.The method of claim 1, wherein said re-oxidizing said initial nitridelayer further comprises: rapidly heating said initial nitride layer inthe presence of oxygen (O₂) gas.
 6. The method of claim 1, wherein saidoxynitride layer has a nitrogen atom concentration of about 1.0×10¹⁵atoms/cm² to about 6.0×10¹⁵ atoms/cm².
 7. A method for forming anitrogen-doped, ultra thin gate dielectric, the method comprising:preparing a hydrogen-terminated, oxide-free silicon substrate surfacefor formation of the dielectric thereupon; forming an initial nitridelayer upon said substrate surface by rapidly heating said substratesurface in the presence of an ammonia (NH₃) gas; re-oxidizing saidinitial nitride layer by rapidly heating said initial nitride layer inthe presence of a nitric oxide (NO) gas, thereby forming an oxynitridelayer; wherein said wherein said oxynitride layer has a nitrogenconcentration therein of at least 1.0×10¹⁵ atoms/cm².
 8. The method ofclaim 7, wherein said oxynitride layer has a thickness of about 5 Å toabout 20 Å.
 9. The method of claim 7, wherein: said forming an initialnitride layer is carried out at temperature of about 650° C. to about1000° C., and at a pressure of about 1 Torr to about 760 Torr.
 10. Themethod of claim 7, wherein: said re-oxidizing an initial nitride layeris carried out at temperature of about 650° C. to about 1000° C., and ata pressure of about 1 Torr to about 760 Torr.
 11. The method of claim 7,wherein said oxynitride layer has a nitrogen atom concentration of about1.0×10¹⁵ atoms/cm² to about 6.0×10¹⁵ atoms/cm².